November 5, 2021 – Increasingly extreme and frequent heat waves are clear signals of the threat to human health posed by climate change, but heat is not the only major factor. High humidity increases the dangers of extreme heat, and days with high humidity also increase.
New evidence published in Geophysical Research Letters shows that both dry and humid heat extremes have increased on the planet. The increases are similar in many regions, including Europe, northern South America, Africa, and most regions of North America. More densely populated areas show the strongest growth on hot and humid days.
On average, every person worldwide has had 5 additional days of extremely humid heat per decade since 1979. If the calculation is based on land area instead of per capita, the increase has been less than 3.5 days since 1979. Extreme dry heat, on the other hand, has occurred around 4 additional days per decade worldwide, regardless of population density.
The people who are hit hardest on these particularly hot and humid days are often hotter than the rest of the world. Extreme dry heat increased particularly in subtropical and desert areas such as the Middle East and Australia. Extreme humid heat occurred where temperatures and humidity were already dangerous, including northern India, parts of Southeast Asia, and parts of Bolivia and Brazil that border the Amazon rainforest.
In these areas, many people rely on farming and other outdoor work, such as construction, and on human-operated transportation such as rickshaws. The increasing pace of extreme heat and moisture events can ruin crops, skyrocket heat-related diseases and prevent working outdoors, all of which threaten productivity in regions with troubled economies.
Precipitation patterns likely play a role in these trends, but a human factor may be irrigation for agriculture. While this research does not offer solutions, it does show the importance of identifying the causes of these extremes and how they affect people in the hardest hit areas.
The researchers write that in these regions, outdoor workers, people without shelter, older adults, and those who live without air conditioning or extreme heat warning systems are most at risk.